On Thu, Sep 01, 2016 at 09:01:41PM +0200, Peter Zijlstra wrote:
> > test_and_set_bit() implies mb() so
> > the lockless list_empty_careful() case is fine, we can not miss the
> > condition if we race with unlock_page().
>
> You're talking about this ordering?:
>
> finish_wait() clear_bit_unlock();
> list_empty_careful()
>
> /* MB implied */ smp_mb__after_atomic();
> test_and_set_bit() wake_up_page()
> ...
> autoremove_wake_function()
> list_del_init();
>
>
> That could do with spelling out I feel.. :-)
This ^^^
> > I am not sure we even want to conditionalize both finish_wait()'s,
> > we could simply call it unconditionally and once before test_and_set(),
> > the spurious wakeup is unlikely case.
>
>
> ret = 0;
>
> for (;;) {
> prepare_to_wait_exclusive(wq, &q->wait, mode);
>
> if (test_bit(&q->key.bit_nr, &q->key.flag))
> ret = action(&q->key, mode);
>
> if (!test_and_set_bit(&q->key.bit_nr, &q->key.flag)) {
> /* we got the lock anyway, ignore the signal */
> ret = 0;
> break;
> }
>
> if (ret)
> break;
> }
> finish_wait(wq, &q->wait);
>
> return ret;
>
>
> Would not that work too?
Nope, because we need to do that finish_wait() before
test_and_set_bit()..
Also the problem with doing finish_wait() unconditionally would be
destroying the FIFO order. With a bit of bad luck you'd get starvation
cases :/
